S. V. Chalov

Synopsis of the interstellar He parameters from combined neutral gas, pickup ion and UV scattering observations and related consequences

A coordinated effort to combine all three methods that are used to determine the physical parameters of interstellar gas in the heliosphere has been undertaken. In order to arrive at a consistent parameter set that agrees with the observations of neutral gas, pickup ions and UV backscattering we have combined data sets from coordinated observation campaigns over three years from 1998 through 2000. The key observations include pickup ions with ACE and Ulysses SWICS, neutral atoms with Ulysses GAS, as well as UV backscattering at the He focusing cone close to the Sun with SOHO UVCS and at I AU with EUVE. For the first time also the solar EUV irradiance that is responsible for photo ionization was monitored with SOHO CELIAS SEM, and the He I 58.4 nm line that illuminates He was observed simultaneously with SOHO SUMER. The solar wind conditions were monitored with SOHO, ACE, and WIND. Based on these data the modeling of the interstellar gas and its secondary products in the heliosphere has resulted in a consistent set of interstellar He parameters with much reduced uncertainties, which satisfy all observations, even extended to earlier data sets. It was also established that a substantial ionization in addition to photo ionization, most likely electron impact, is required, with increasing relative importance closer to the Sun. Furthermore, the total combined ionization rate varies significantly with solar latitude, requiring a fully three dimensional and time dependent treatment of the problem.

Scatter-free pickup ions beyond the heliopause as a model for the interstellar boundary explorer ribbon

We present a new kinetic-gasdynamic model of the solar wind interaction with the local interstellar medium. The model incorporates several processes suggested earlier for the origin of the ribbon?the most prominent feature seen in the all-sky maps of heliospheric energetic neutral atoms (ENAs) discovered by the Interstellar Boundary Explorer (IBEX). The ribbon is a region of enhanced fluxes of ENAs crossing almost the entire sky. Soon after the ribbons discovery, it was realized that the enhancement of the fluxes could be in the directions where the radial component of the interstellar magnetic field around the heliopause is close to zero. Our model includes secondary charge exchange of the interstellar H atoms with the interstellar pickup protons outside the heliopause. Previously, in the frame of a kinetic-gasdynamic model where pickup protons are treated as a separate kinetic component, it was shown that the interstellar pickup protons outside the heliopause may be a significant source of ENAs at energies above 1 keV. The key difference between the current work and the previous models is in the assumption of no pitch-angle scattering for newly created pickup protons outside the heliopause. We demonstrate that in the limit of no pitch-angle scattering ribbon of enhanced ENA fluxes appears in the model, and this may qualitatively explain the ribbon discovered by IBEX.

The dynamical role of anomalous cosmic rays in the outer heliosphere

The two-dimensional model of the solar wind-local interstellar medium interaction developed by Baranov & Malama (1993, J. Geophys. Res., 98, 15 157) is improved by taking into account the dynamical effect of anomalous cosmic rays. The cosmic rays are treated as a massless diffusive fluid. It is shown that the termination shock shifts further away from the Sun in this case and the post-shock temperature of the thermal plasma decreases. The magnitude of the shift depends on the value of the energy-averaged spatial diffusion coefficient. The number density of energetic neutral hydrogen originating in the shocked solar wind is generally lower in the case of the cosmic-ray-modified termination shock.

Self‐consistent model of the solar wind interaction with three‐component circumsolar interstellar cloud: Mutual influence of thermal plasma, galactic cosmic rays, and H atoms

In this paper we continue our study of the galactic cosmic ray (GCR) influence on the structure of the heliospheric interface plasma flow [Myasnikov et al., this issue]. The model presented here is more realistic and takes the mutual influence of plasma, neutral, and cosmic ray components into account self-consistently. In the model, GCRs are described hydrodynamically under the assumption that their mass density is negligible; while neutrals are described kinetically. We explore the GCR influence on the heliospheric interface plasma structure by varying the diffusion coefficient, cosmic ray pressure, and adiabatic index. The problem is studied numerically, using the global iterations that couple the soft fitting technique for describing the plasma and GCR components and Monte Carlo simulations for H atoms. A strong GCR modulation is found in the heliospheric interface. At the same time, the GCR influence on the plasma flow is negligible as compared with the influence of H atoms. The exception is the bow shock, a structure which can be strongly modified by the cosmic rays. The Baranov-Malama model is therefore acceptable for interpretation of the physical processes in the heliosphere as long as the processes are not related to the bow shock structure. Although the simplest model of the cosmic ray transport is good enough to estimate GCR influence on the plasma and atom distributions in the heliospheric interface, more advanced models should be used to interpret the observed GCR spectra.

Reabsorption of self-generated turbulent energy by pick-up protons in the outer heliosphere

It is well known that newly created pick-up ions are first injected into a toroidal distribution in the velocity space which comoves with the solar wind. This distribution is, however, unstable with respect to excitation of resonant MHD waves, and thus by wave-driving some specific amount of the initial kinetic energy of pick-up ions is handed over to the ambient wave fields, while the pick-up ion velocity distribution thereby is rearranged. In this paper we describe the developement of the wavenumber spectrum of pick-up proton-generated turbulence and the pick-up proton energy spectrum resulting from resonant reabsorption of this turbulence, by solving simultaneously the coupled system of two differential equations for the spectral turbulence power and the pick-up proton spectral intensity. We show that the majority of pick-up generated wave energy in the range of wavenumbers above the wavenumber at which the waves are produced is reabsorbed by the pick-up protons themselves.

Supersonic solar wind ion flows downstream of the termination shock explained by a two-fluid shock model

In the view of classic magnetohydrodynamics, it is generally expected that at a shock the supersonic upstream flow is converted into the subsonic downstream flow whereby the increased entropy appears in the increased downstream temperature. In the solar wind termination shock, it is expected that the supersonic upstream solar wind ion flow changes into a subsonic downstream ion flow. The Voyager-2 passage over this shock, however, demonstrated that the downstream solar wind ion flow still has a supersonic signature. In this paper, we present straightforward solution to this unexpected phenomenon by applying a two-fluid model to describe the passage of the solar wind over the termination shock. The two dynamically dependent, but thermodynamically independent fluids are the normal solar wind ions and the comoving suprathermal ions, or so-called pick-up ions. As we can show, the downstream solar wind ion flow can be either subsonic or supersonic depending on the upstream effective Mach number of the flow or the pick-up ion pressure. This is because most of the kinetic energy of the upstream flow is converted into thermal energy of the suprathermal ions, while the normal solar wind ions are heated only ineffectively, so that they can retain a supersonic signature even further downstream. With this two-fluid model, we can explain the main features of the shock structure observed by Voyager-2.

Self-consistent model of the solar wind interaction with two-component circumsolar interstellar cloud: Mutual influence of thermal plasma and galactic cosmic rays

The interaction of the solar wind with the two-component interstellar medium consisting of thermal plasma and galactic cosmic rays is studied numerically using a soft fitting technique based on the high-resolution Godunov scheme and the physical process splitting method. Mutual influence of plasma and cosmic ray components on the solar wind termination is considered. It is shown that cosmic rays can considerably modify the shape and structure of the solar wind termination shock and the bow shock, and change the positions of the heliopause and bow shock if no H atoms are taken into account.

Energetic particles from the outer heliosphere appearing as a secondary pick-up ion component

In measured pick-up ion spectra a special spectral feature has been identified on the high side of the pick-up ion injection energy which cannot be explained as due to energy diffusion of primary pick-up protons originating from ionized interstellar H-atoms. As we can show, however, in the following article, this feature can be explained as a secondary pick-up ion feature due to ions originating from the charge exchange ionization of energetic neutral H-atoms entering the inner heliosphere and coming from their birth places in the heliosheath. It may thus be the case that energetic neutral H-atoms from the heliosheath region, searched for by experimenters for quite some time now, have already been seen as secondary pick-up ions in special features of measured distribution functions.

Different solar wind types reflected in pick-up ion spectral signatures

In the more recent years several papers have been published which presented theoretical results concerning the phase-space behaviour of pick-up ions (PUIs) after their injection into the supersonic solar wind. PUI spectra thereby were obtained which revealed detailed spectral properties that could well be compared with observational data. Effects of convection, adiabatic deceleration, pitch-angle diffusion, and even of energy diffusion could clearly be identified. It was, however, becoming evident in the data that specific spectral differences between pick-up spectra in slow solar wind streams and in fast solar wind streams can be found. For instance, the high-energy shoulders of pick-up spectra beyond the injection energy were less pronounced in fast compared to slow solar wind streams. In order to come up with a more quantitative theoretical representation of this phenomenon we shall study in this paper the specifically different conditions for PUI phase-space transports in fast and in slow solar wind streams taking into account the type-specific main quantities governing pick-up ion phase-space motions like the injection velocities, the turbulence levels, and the exposure times.

On the Origin of Energetic Neutral Atoms Detected by the SOHO/CELIAS/HSTOF Sensor

We summarize observational data and present theoretical considerations in order to estimate the relative contributions of suprathermal pick‐up ions neutralized inside and outside the termination shock to the energetic neutral atom (ENA) flux in the energy range 55 – 80 keV/amu detected by the SOHO/CELIAS/HSTOF sensor near Earth’s orbit.